Method for removing sulfur dioxide from gas streams

ABSTRACT

A process and apparatus for removing SO 2  from low-volume gas streams containing high concentrations of SO 2  includes a tank scrubber containing an aqueous SO 2  absorbent and an impeller. The impeller includes a vertical rotating shaft having an upper and lower impeller. The lower impeller serves to agitate the SO 2  absorbent. The upper impeller includes a shroud thereby directing the absorbent and gas outwardly toward the wall of the scrubber. The gas is injected into the absorbent at a point below the shrouded impeller. The shroud prevents gas bypass and thereby promotes gas-liquid mixing and, consequently, markedly improves SO 2  removal efficiency.

FIELD OF THE INVENTION

The present invention relates to a process and apparatus for scrubbingsulfur dioxide from SO₂ -containing gas streams hereinafter referred toas flue gases or simply as gases. More specifically, it relates to adispersed-gas phase sulfur dioxide scrubber or tank scrubber providedwith an impeller means to disperse the SO₂ -containing gas in an aqueousabsorbent.

BACKGROUND OF THE INVENTION

Large amounts of sulfur dioxide are emitted annually by industrialplants worldwide and strict legislation to control these emissions isbeing enacted in most countries. Wet scrubbing systems, such as spraytowers, are the most common means employed for removing sulfur dioxidefrom flue gas. The wet scrubbers typically use an aqueous solution orslurry of an alkaline reagent, such as sodium bicarbonate, sodiumcarbonate, lime or limestone, to remove the SO₂ from the flue gas. Thesesolutions and slurries of alkaline reagents are often referred to as"SO₂ absorbent" or simply as "absorbent." While the conventional wetscrubbers have proved to be a fairly effective means of removing SO₂,they tend to have high maintenance costs due to excessive scalebuild-up. Additionally, SO₂ removal efficiency of the conventional wetscrubbers is only about 90%, which is not always adequate to enable theemitted flue gas to meet regulatory standards in many areas around theworld.

Tank scrubbers offer an alternative to the conventional wet scrubbers.The tank scrubbers generally provide a vessel containing an aqueousslurry or solution of an alkaline reagent, usually either lime orlimestone, to serve as the absorbent for SO₂. The flue gas is injecteddirectly into the absorbent slurry. Several devices have been proposedto contact the SO₂ -containing gas with the absorbent slurry. Forexample, in U.S. Pat. No. 4,099,925, the flue gas is contacted with theabsorption slurry in a liquid-raising pipe. In U.S. Pat. No. 4,156,712the flue gas is sparged into an upper portion of the liquid absorbentwithout the aid of any mechanical agitation above the level of flue gasinjection. In U.S. Pat. No. 4,229,417, flue gas dispersion is achievedin a manner similar to that of the '712 patent except that the pipesfeeding the flue gas into the absorbent slurry have notches to providefor better gas dispersion. U.S. Pat. No. 4,911,901 discloses a processwherein the flue gas is contacted with the spray of the absorbent slurryprior to gas-slurry contact in the reactor. The SO₂ removal efficienciesattained by these prior art tank scrubbers are in the range of 90-99%which is a considerable improvement over conventional wet scrubberefficiencies. The efficiency of these tank scrubbers is still not alwaysadequate to meet regulatory requirements, particularly in cases wherethe SO₂ concentration in the flue gas is relatively high, and moreparticularly where the gas contains greater than 1% SO₂.

Tank scrubbers generally include a device to bubble the flue gas throughthe absorbent. As a result, the tank scrubbers typically are limitedwith respect to the volume of flue gas that can be treated.

Although the current use of tank scrubbers has been limited due to theirdifficulty in handling very large volumes of flue gas, there are manypotential applications for the tank scrubbers in industrial plants whichproduce relatively small volumes, i.e., less than 25,000 CFM, of fluegas with a high SO₂ content. For example, newly developed metallurgicalprocesses, such as oxygen-enriched gold ore roasting, produce suchgases. Small Claus plants also produce such gases. Due to the highefficiency of the tank scrubbers when used to treat low volumes of fluegases, the number of potential applications should grow as environmentalregulations become increasingly stringent. Continued improvements intank scrubber design would provide even more opportunities for the useof the tank scrubbers.

There is therefore a continual need for improving the efficiency of tankscrubbers to remove SO₂ from flue gas and other gas streams. There isfurther a particular need for improved tank scrubbers that are able toremove large amounts of SO₂ from low-volume streams of flue gascontaining high concentrations of SO₂.

The present invention is accordingly directed to a method and apparatusfor providing an improved tank scrubber design for removing SO₂ fromflue gas. The process and apparatus of the invention is able to removeSO₂ effectively from low-volume streams of flue gas containing highconcentrations of SO₂.

SUMMARY OF THE INVENTION

The present invention is directed to a process and apparatus forscrubbing sulfur dioxide from SO₂ -containing gases and particularlyflue gases containing sulfur dioxide in amounts greater than 1% byvolume. The SO₂ -containing gas, which contains SO₂ and preferably alsoO₂, is compressed to a pressure of about 2-8 psig and injected into atank scrubber also fed with a fresh SO₂ absorbent. Suitable absorbentsinclude, for example, aqueous solutions or aqueous slurries of sodiumbicarbonate, sodium carbonate, or sodium hydroxide, and aqueous slurriesof lime and/or limestone, and mixtures thereof. The preferred absorbentis an aqueous slurry of lime. The tank scrubber includes an agitatingmeans to disperse the gases in the absorbent, to circulate the aqueousabsorbent and to maintain solids in suspension. The agitation system ofthe tank scrubber includes an upper impeller means designed to providehigh shear and a lower impeller means designed to provide agitation. Theupper and lower impellers are preferably mounted on a single rotatingimpeller shaft positioned in the tank such that the impellers are belowthe surface of the absorbent. In another embodiment, the impellers maybe mounted on separate rotating shafts. In a preferred embodiment, thelower impeller is a pitched blade. Preferably, the upper impeller is ashrouded flat-blade disk design including a circular disk, which servesas a shroud, and a plurality of radially extending flat blades or vanesterminating at the outer edge of the disk. The flat blades are fixed tothe underside of the disk. The shroud of the upper impeller ispreferably located between one and three feet below the surface of theabsorbent solution or slurry. The SO₂ -containing gas is injected intothe solution or slurry of SO₂ absorbent and dispersed in said solutionor slurry by the upper impeller. The pH of the aqueous absorbent ispreferably maintained at about pH 4.5 to pH 7.0 by the addition of freshabsorbent. The solution or slurry temperature preferably is maintainedat 33°-185° F. The SO₂ -containing gas is injected at a point near theimpeller shaft between the upper and lower impellers at 3 to 5 feetbelow the surface of the absorbent solution or slurry held in the tankscrubber. A single nozzle gas injection means is preferably used toinject the SO₂ -containing gas in a downward direction. Alternatively, amultiple injection nozzle may be used.

The SO₂ contained in the gas dissolved in the aqueous phase of theabsorbent and reacts with the absorbent to form intermediate products,such as sodium bisulfite NaHSO₃, sodium sulfite Na₂ SO₃, calciumbisulfite Ca(HSO₃)₂, and/or calcium sulfite CaSO₃. If sufficient oxygenis available, the corresponding sulfates are formed. If insufficientoxygen is available in the injected gas to complete the reaction to Na₂SO₄ or CaSO₄, additional oxygen may be added either into the gas inletline, directly into the tank scrubber solution or slurry or directlyinto the solution or slurry effluent line from the tank scrubbers.

In one preferred embodiment of the invention, the process is carried outas a continuous process. The fresh aqueous absorbent is continuouslyintroduced to the tank scrubber whereby the absorbent in the tankcomprises a mixture of fresh absorbent and spent absorbent. The freshabsorbent is introduced to the tank at a rate complementing theinjection of the SO₂ -containing gas whereby the pH of the aqueousabsorbent is preferably maintained between about pH 4.5 and pH 7.0 andmore preferably between about pH 5.4 to 6.0.

The spent SO₂ absorbent is withdrawn from the agitated tank at a ratesufficient to maintain a constant solution or slurry level at about 1 to3 feet above the top of the shrouded impeller.

BRIEF DESCRIPTION OF THE DRAWINGS

The description of the invention is to be considered in combination withthe drawings of which the following is a brief description.

FIG. 1 is schematic diagram of the apparatus for carrying out theprocess in a preferred embodiment of the invention.

FIG. 2 is a bottom perspective view of the shrouded impeller accordingto a preferred form of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The disadvantages and limitations of the previous tank scrubbers areobviated by the process and apparatus of the present invention. Theinvention is directed to a process of removing sulfur dioxide from gasesand particularly flue gases using a novel tank scrubber. The process ofthe invention is able to effectively remove large amounts of sulfurdioxide from gases containing from 1% to as much as 50% by volume sulfurdioxide.

An SO₂ -containing gas, such as flue gas, is injected into a tankscrubber containing an SO₂ absorbent. The tank scrubber includes animpeller means comprising an upper shrouded high shear impeller. Theimpeller means further includes a lower pitched blade serving to agitatethe aqueous absorbent. The upper and lower impellers are preferablymounted on a common vertical rotating shaft having an upper and lowerzone for mounting said impellers. The impellers may also be mounted onseparate shafts. The SO₂ -containing gas is injected into the aqueousabsorbent at a point between the upper and lower impellers preferablyabout 3 to 5 feet below the level of the aqueous absorbent. The upperimpeller provides shearing action to disperse the injected gas and toaccelerate the absorption of SO₂ into the absorbent. The shroud on theupper impeller substantially prevents the SO₂ -containing gas frombubbling out of the absorbent before being dispersed in the absorbent.The blades of the lower impeller provide agitating action and may bepitched so as to lift the absorbent along the shaft or, alternatively,to propel the absorbent toward the bottom of the tank.

In the preferred embodiments of the invention the SO₂ absorbent is anaqueous solution or slurry of an alkaline absorbent material. Thepreferred alkaline absorbent material is calcium carbonate such aslimestone, calcium oxide such as lime and calcium hydroxide such asslaked lime. Other satisfactory alkaline absorbent materials mayinclude, for example, sodium bicarbonate, sodium carbonate, sodiumhydroxide.

The process of the invention is preferably a continuous process suchthat fresh aqueous absorbent is continuously added to the reactionvessel and spent absorbent is continuously removed. Although lesspreferred, the process may be carried out in a batch process. The SO₂-containing gas is injected into the absorbent as described in greaterdetail hereinafter. Throughout the SO₂ -containing gas injection andaddition of fresh absorbent, the pH of the aqueous absorbent in the tankscrubber is preferably maintained between about 4.5 and 7.0 and morepreferably about 5.4 to 6.0. Under certain operating conditions, the pHof the absorbent may be allowed to rise above 7.0. The pH of the aqueousabsorbent is preferably controlled by coordinating the rate of the freshabsorbent addition with the rate of the gas injection such that the pHis maintained at the desired level. The amount and rate of the additionof fresh absorbent is dependent on the concentration of the absorbentand the sulfur dioxide content of the gas being treated.

The apparatus according to a preferred embodiment of the invention isshown in FIG. 1. Tank scrubber 4 is a closed-top vessel equipped with anagitation system consisting of an upper shrouded impeller 6 to providehigh shear and a lower pitched-blade impeller 8 to provide agitation.Preferably, the shrouded impeller 6 is provided with four or six flatrectangular blades 20 for gas dispersion. The tips of the blades asshown in FIG. 1 are aligned with the outer edge of the impeller shroud7. The size of the impeller will vary with the size of the tank andother factors. For example, a 20 foot diameter tank will typicallyrequire about a 6 to 7 foot diameter upper impeller and about a 9 to 10foot diameter lower impeller to achieve maximum efficiency. The shroud 7prevents by-passing of the injected gas along the impeller shaft 5 andallows for the injection of gas at relatively shallow depths in theaqueous absorbent without hindering the flue gas retention time in theabsorbent. Shallow gas injection reduces the horsepower requirements forgas injection. The upper and lower impellers 6 and 8 are preferablyattached to a common impeller shaft 5 driven by a motor 9.

The tank scrubber solution or slurry 10, hereinafter also referred to asthe "tank scrubber fluid" or simply as the "fluid", maintained in tankscrubber 4 comprises an aqueous mixture of fresh and spent SO₂absorbent. The tank scrubber fluid 10 is maintained at a pH above 4.5and preferably above 5.4 by the addition of fresh absorbent. When anaqueous slurry of lime is used as the fresh absorbent, the tank scrubberfluid 10 is preferably maintained at a pH range of 4.5-7.0 and morepreferably at a pH range of 5.4-6.0. Additional fresh SO₂ absorbent iscontinually added via line 13 to adjust the pH of the tank scrubberfluid 10. The pH of the tank scrubber fluid may be allowed to rise above7.0, however, at pH values above 7.0, the reaction occurring betweenCa(OH)₂ and the carbon dioxide contained in the flue gas causesincreased CaCO₃ formation and might result in an increased tendency toscale and in higher lime consumption. At pH values below 4.5, the SO₂solubility becomes so low that scrubber efficiency is detrimentallyaffected. The tank scrubber fluid surface level 11 is preferablymaintained at about 1 to 3 feet above the top of the shrouded impeller6. The scrubber fluid surface level 11 is controlled by continual orperiodic withdrawal of slurry contained in the tank scrubber 4. Tankscrubber 4 is dimensioned to provide a gas collection space 12 above theabsorbent fluid to collect the treated gases prior to leaving thescrubber 4 via line 14.

Referring to FIG. 1, flue gas which contains SO₂ and preferably also O₂,and which has been compressed to about 2-8 psig, enters tank scrubber 4via lines 1 and 2 and injection nozzle 3. As shown in FIG. 1 the fluegas injection nozzle 3 preferably injects the flue gas downwardly in thedirection toward the lower impeller 8. The flue gas is injected beneaththe surface of tank scrubber fluid 10 at a point near the agitator shaft5 directly beneath the upper shrouded impeller 6. This flue gasinjection point is preferably about 3 to 5 feet below the tank scrubberfluid surface level 11. Preferably, a single injection nozzle 3 is usedto avoid potential scale buildup that commonly results from the use ofmultiple injection points. The interior of the injection nozzle 3 may bekept clean by the action of multiple water sprays (not shown) directedagainst the interior nozzle lip and inner nozzle pipe surface.

The SO₂ in the flue gas reacts with the absorbent contained in the tankscrubber fluid 10 to form the intermediate reaction products such assodium bisulfite NaHSO₃, sodium sulfite Na₂ SO₃, calcium bisulfiteCa(HSO₃)₂ and/or calcium sulfite CaSO₃. These intermediate productsfurther react with oxygen to form the corresponding sulfates whensufficient oxygen is present in the scrubber. If there is insufficientO₂, i.e. less than about 5% O₂ by volume, available in the flue gas toefficiently complete the oxidation of the intermediate product, an O₂source may be added. In the preferred embodiments of the invention theflue gas contains at least about 5% by volume oxygen at the time it isinjected into the absorbent. This additional O₂ source can be added tothe flue gas inlet line 1 by means of line 18. The additional O₂ sourcecould also be added directly to the tank scrubber fluid 10 through adistribution system such as for example nozzle 19 which could be locatedbelow the surface 11 of the tank scrubber fluid 10 adjacent the flue gasinjection nozzle 3 and below the upper impeller 6. In another mode, theeffluent tank scrubber fluid 10 exiting tank scrubber 4 by lines 15 and17 and valve 16 may be treated with air or oxygen by means external totank scrubber 4 (not shown). The oxygen added to the flue gas tosupplement the oxygen content of the gas is generally in the form of anoxygen-containing gas, such as air, although a more pure or concentratedform of oxygen may be used.

The tank scrubber 4 is sized to provide a fluid retention time of atleast 30 minutes, and preferably between 8 and 24 hours. The tankscrubber fluid 10, which is maintained at 33°-185° F., is withdrawn fromtank scrubber 4 via line 15, control valve 16 and line 17. If a sodiumbicarbonate or sodium carbonate solution is used as the absorbent, theresulting sodium sulfate solution can be regenerated using lime as knownin the art. If lime is used as the absorbent as in the preferredembodiment, the tank scrubber fluid 10 can be sent to a tailings pondfor disposal or, alternatively, the fluid can be dewatered and itssolids content recovered for use in industrial products.

A unique advantageous feature of the present tank scrubber 4 is theagitation system. The upper shroud 7, by preventing the short circuitingof gas up the agitator shaft 5, allows for relatively shallow injectionof the flue gas and still facilitates excellent gas-fluid contact in aminimal volume of adsorbent. This minimizes flue gas compression needs,i.e. about 2 to 8 psig, and saves considerably on power requirements.The shroud 7 in combination with the two impeller means provides intenseagitation and mixing of gases and tank scrubber fluid 10 and,consequently promotes the rapid and efficient absorption of SO₂ by thetank scrubber fluid 10.

FIG. 2 is a bottom perspective view of the shrouded impeller 6 and theshroud 7. In this embodiment, the impeller has six flat rectangularblades 20 mounted directly beneath the shroud 7. As shown, the shroud 7is a substantially flat disk-like member fixed to the impeller shaft 5.The blades 20 are substantially of a rectangular shape fixed to thebottom side of the shroud 7. The blades 20 are preferably positioned ina vertical plane substantially parallel with the axis of the impellershaft 5. In the embodiment shown in FIG. 2 the impeller blades extendradially outward from the impeller shaft 5 and terminate at theperipheral edge of the shroud 7. Preferably the blades 20 are spacedslightly from the shaft 5. Alternatively the blades 20 may contact theshaft 5 and extend radially outward to the peripheral edge of the shroud7. In a further embodiment the impeller blades 20 may be pitched withrespect to the axis of the impeller shaft 5.

The shroud 7 and the impeller blades 20 are preferably dimensioned toprevent the injected flue gas from passing upwardly along the impellershaft and prematurely exiting the scrubber. The shaft 5 is rotated at aspeed sufficient to direct the flue gases radially outward and todisperse the flue gases in the absorbent and promote sufficientretention time to effectively remove the sulfur dioxide.

The principal advantage of the present invention is the efficiency ofSO₂ removal from the flue gas. Prior art tank scrubbers provide a rangeof SO₂ removal efficiency of 90-99% and, more typically, provide about93-96% SO₂ removal efficiency for flue gases containing comparativelylow levels of sulfur dioxide. In contrast, the apparatus and process ofthe present invention provide an SO₂ removal efficiency in the order of99.9%, based on full scale plant operational data, which is one order ofmagnitude better than the highest previously obtained. The process andapparatus of the invention has been shown to be effective in removingsulfur dioxide from flue gases containing high levels of sulfur dioxideas well as other contaminants.

Other advantages include simplicity of design. For example, the flue gasis injected into the tank scrubber fluid 10 via a single injectionnozzle 3 while prior art processes typically utilize multiple injectionpipes. Based on experience derived from full scale plant operation, thesimplicity of design translates into low maintenance costs and ease ofoperation. The ability of the present device to use a single injectionnozzle rather than a plurality of nozzles is due to the efficiency ofthe shrouded impeller to effectively disperse the flue gas in theabsorbent.

It is another distinct advantage of the present invention that amechanical method of creating the gas dispersion is provided. Theprocess of the invention utilizes a shrouded impeller 6 to create a finedispersion of the gas in the tank scrubber fluid 10 and therebyfacilitates efficient removal of SO₂ from the flue gas.

It is still another advantage of the invention that scale formation ontank equipment is largely eliminated in systems using calcium compoundssuch as lime as the SO₂ absorbent. In the present embodiment of theinvention, this is achieved because calcium sulfite crystals tend togrow preferentially on the finely dispersed solids produced by theinnovative techniques employed. Other advantages of the invention willbecome obvious to those skilled in the art.

In one preferred embodiment a single tank is used. A second holding tankor scrubber tank may be used in conjunction with the first tank toincrease the retention time thereby allowing growth to occur properly.

The present system is particularly suitable to handle the discharge gasfrom a gold ore, oxygen-enriched roasting operation such as thatdescribed in U.S. Pat. No. 4,919,715. About 2600 SCFM of flue gas withan SO₂ content of 4-14% and an O₂ content of about 15-17% are emittedfrom the gold ore roasting operation. Operating efficiencies for thetank scrubber unit during full-scale roasting plant operations haveexceeded 99.9%. The novel tank scrubber of the invention is ideallysuited for other oxygen-enriched, gold ore roasting plants and, also,for similar-sized plants in other industries, particularly those whichemit relatively small volumes of high SO₂ -content flue gas, forexample, Claus plants. The presence of substantial quantities of O₂ inthe flue gas is also beneficial although, as previously discussed, anoxygen-providing gas can be injected separately, as required.

EXAMPLE

A 2660 SCFM stream of flue gas from a gold ore oxygen roasting plantcontaining 4% SO₂ and 17% O₂ was fed into a tank scrubber as shown inFIG. 1. The temperature and pressure of the flue gas stream was 160° F.and 3.8 psig, respectively. The tank scrubber was 20 ft in diameter by22 ft high. The tank scrubber was equipped with a shrouded impeller,having a shroud 82 inches in diameter and six blades each 38 inches longby 12.2 inches high, welded to the shroud so that the outer tips of theblades are aligned with the outer edge of the shroud. Mounted on thesame shaft below the shrouded impeller was a pitch blade impeller 112inches in diameter. The relative position of the impellers was about 144inches and 37 inches from the bottom of the tank, respectively. Theimpellers were equipped with a 75 hp. motor and were operated at about37 RPM. The tank contained a 20% by weight calcium sulfate slurry, thepH of which was maintained in the range of 5.4 to 6.0 by the continuousaddition of a lime slurry. The slurry level in the tank scrubber wascontrolled at about 15 ft. The excess slurry was continuously withdrawnfrom the tank scrubber to maintain the slurry at a constant level. Thegas stream exiting the scrubber contained about 20 ppm SO₂, equivalentto an SO₂ removal efficiency of about 99.95%.

What is claimed is:
 1. A process for removing SO₂ from an SO₂-containing gas comprising:a. adding an aqueous SO₂ absorbent into avessel in an amount to maintain an SO₂ absorbent fluid at a pH of atleast 4.5; b. agitating said aqueous SO₂ absorbent fluid utilizing arotating impeller comprising an upper impeller and a lower impeller,wherein the upper impeller includes at least one shroud to inhibit thebypassing of gas; c. injecting SO₂ -containing gas below the surface ofsaid aqueous absorbent fluid to a proximity between the said upper andlower impellers; d. maintaining the level of the aqueous absorbent fluidin said vessel at a predetermined height by withdrawing aqueousabsorbent fluid; and e. removing desulfurized gas from said vessel. 2.The process of claim 1 wherein the SO₂ absorbent fluid is an aqueoussolution or slurry of an alkaline component selected from the groupconsisting of sodium bicarbonate, sodium carbonate, sodium hydroxide,slaked lime, limestone and mixtures thereof.
 3. The process of claim 1wherein the SO₂ content of the gas entering the vessel is about 1% toabout 50% by volume.
 4. The process of claim 1 wherein the gas enteringthe vessel contains oxygen and wherein its oxygen content is at leastabout 5% by volume.
 5. The process of claim 1 comprising adding oxygento said gas prior to injecting into said absorbent fluid to provide atotal O₂ content in the gas of at least about 5% by volume.
 6. Theprocess of claim 1 comprising adding an oxygen-containing source to saidabsorbent fluid in an amount sufficient to oxidize substantially allsulfite and bisulfite products in the aqueous absorbent fluid to asulfate.
 7. The process of claim 1 comprising injecting oxygen into theaqueous absorbent fluid withdrawn from said vessel in step (d) tooxidize substantially all sulfite and bisulfite products to a sulfate.8. The process of claim 1 comprising compressing said gas to about 2-8psig prior to injecting into the aqueous absorbent fluid.
 9. The processof claim 1 wherein the temperature of the fluid is maintained at about33°-185° F.
 10. The process of claim 1 wherein the SO₂ absorbent fluidis a slurry of lime.
 11. The process of claim 1 wherein the pH of theaqueous absorbent fluid in said vessel is maintained at about 5.4-6.0.12. The process of claim 1 comprising retaining the aqueous absorbentfluid in the vessel for at least 30 minutes.
 13. The process of claim 1comprising retaining the aqueous absorbent fluid in the vessel for about8-24 hours.
 14. The process of claim 1 comprising maintaining the levelof the aqueous absorbent fluid at about 1 to 3 feet above the top of theupper impeller.
 15. The process of claim 1 comprising injecting the gasat about 3-5 feet below the surface of the absorbent fluid.
 16. Theprocess of claim 1 comprising injecting the gas into the aqueousabsorbent fluid through a single nozzle.
 17. The process of claim 1wherein said upper and lower impeller are mounted on a common rotatingshaft.
 18. The process of claim 1 wherein said upper and lower impellerare each mounted on a separate rotating shafts.
 19. The process of claim1 comprising rotating said upper impeller at a speed to producesufficient shear to disperse said SO₂ -containing gas.
 20. The processof claim 1 wherein said lower impeller comprises a pitched bladeimpeller to agitate said aqueous absorbent.
 21. The process of claim 20wherein said upper impeller comprises a substantially horizontalcircular disk fixed to a substantially vertical rotating shaft and aplurality of radially extending vanes disposed on a lower side of saiddisk and extending from said shaft to a peripheral edge of said disk.22. The process of claim 21 comprising injecting an oxygen-containinggas in proximity to said shaft between said upper and lower impeller.23. A continuous process for removing SO₂ from SO₂ -containing gascomprising the steps of:a. continuously introducing an SO₂ -containinggas into a vessel containing an aqueous SO₂ absorbent, wherein saidaqueous absorbent within said vessel has a pH of at least 4.5, saidvessel including an agitator means comprising a rotating substantiallyvertical shaft having an upper and lower zone, said lower zone having apitched blade first impeller means to agitate said aqueous SO₂absorbent, said upper zone of the shaft including a second impellermeans disposed above said first impeller means and having asubstantially horizontal shroud fixed to said shaft and means to directsaid aqueous absorbent substantially radially outward from the shaft; b.injecting said SO₂ -containing gas into said aqueous absorbent at apoint disposed between said upper and lower impeller means andsimultaneously agitating said aqueous absorbent by rotating saidimpeller shaft at a speed to produce sufficient shear whereby injectedflue gas is directed radially outward to disperse said gas in saidaqueous absorbent whereby absorption of SO₂ from said gas is effected;and c. removing said injected gas from the aqueous absorbent.
 24. Theprocess of claim 23 comprising continuously introducing an aqueousabsorbent to said vessel at a rate whereby said aqueous absorbent in thevessel has pH of at least 4.5.
 25. The process of claim 23 comprisingcontinuously removing spent aqueous absorbent from said vessel wherebythe absorbent is maintained at a substantially constant level.